In addition to coordinating immune and inflammatory responses, NF-kappaB/Rel transcription factors control cell survival. This anti-apoptotic function is crucial to oncogenesis, cancer chemoresistance, and to antagonize TNF-R-induced killing. Using a functional screen, we have identified Gadd4513/Myd 118 as a pivotal mediator of the NF-kappaB protective activity against apoptosis by TNFalpha. Cytoprotection by Gadd45beta involves the inhibition of the JNK cascade, and this inhibition is central to the control of cell death by NF-kappaB. Indeed, Gadd45a expression depends on NF-kappaB, corrects the apoptotic defect of NF-kappaB null cells, and is essential to NF-kappaB-dependent survival. However, the bases for the NF-kappaB protective functions in oncogenesis and resistance to anti-cancer therapy remain poorly understood. Likewise, the step(s) controlled by Gadd45beta during apoptosis, and the mechanisms by which Gadd45beta inhibits JNK signaling are not known. This proposal has three Specific Aims, which will address these issues. In Aim 1, by combining an in vivo and an in vitro approach, we will systematically examine the impact of Gadd45beta on Fas-induced pathways in BJAB cells, focusing on two major checkpoints: the DISC and the mitochondria. We will also assess the relevance of the targeting of the JNK cascade to the control of Fas-induced killing by Gadd45beta. In Aim 2, we will determine the mechanisms by which Gadd45beta suppresses the JNK pathway and the bases for the specific inhibition of this pathway by Gadd45beta. We will begin by assessing the significance of newly discovered interactions between Gadd45beta and components of the JNK cascade. If necessary, by using chromatographic methods, we will identify the additional targets of Gadd451a that are relevant to JNK inhibition and cytoprotection. In Aim 3, by using the Ras transformation and breast cancer model systems, we will address the roles of Gadd45beta and the targeting of the JNK pathway in NF-?B-dependent tumorigenesis. We will also perform a systematic analysis of Gadd45beta expression in primary human tumors, focusing on those tumors that depend on NF-kappaB for their survival. Finally, we examine the significance of the Gadd451a anti-apoptotic activity against chemotherapy drugs to the resistance of tumor cells to anti-cancer treatment. The goal of these studies is to determine whether, like NF-kappaB, Gadd45beta represents a target for cancer therapy.